Conventional television sets and many other display devices generate their displays by using one or more electron guns to illuminate each line on the television screen. The television transmission signals typically include a vertical blanking interval (VBI), which is the time required to reset the electron gun to the top of the television screen from the bottom of the screen after the screen has been illuminating. Often a portion of the VBI is used by television broadcasters to transmit control information necessary for displaying audio and video information. This control information includes, for example, video synchronization signals. In addition, a portion of the VBI is typically reserved for broadcasting the text for closed captioning for the hearing impaired.
Broadcasters also may use time within the VBI to broadcast a variety of other types of data, typically referred to as teletext data, of the broadcaster's own choosing. VBI scan lines are typically broadcast in accordance with one of a number of industry standard formats. For example, teletext scan lines are often broadcast in accordance with the North American Basic Teletext Specification (NABTS). See "Joint EIA/CVCC Recommended Practice for Teletext North American Basic Teletext Specification (NABTS)", EIA-516, May 1988. NABTS calls for a modulated data bit rate of 5.7272 MHz.
Hardware for capturing teletext data commonly samples received teletext data using a technique called "oversampling" to recover the bit peaks of the teletext data. Oversampling may be defined as sampling the received analog signal at a rate that is higher than the teletext data symbol rate (e.g., the NABTS rate). For example, teletext data is typically oversampled at a rate that is from two-to-eight times the NABTS rate.
One problem associated with the broadcast of teletext data is ghosting. Ghosting is often caused by reflections of a television signal off of an object in the signal's path. Ghosting presents a problem when transferring data over the VBI, because the reflected signals interfere with the data that is being transmitted. Accordingly, televisions and computer systems equipped to receive television signals may include an anti-ghosting mechanism. Examples of anti-ghosting mechanisms are Adaptive Baseband Equalization (ABBE) filtering and Lookup Table (LUT) filtering.
A computer system equipped to receive television signals typically includes a tuner subsystem for capturing the television signals, which may be connected on an expansion bus, such as a Peripheral Component Interconnect (PCI) bus. Decoding and anti-ghosting of the teletext data are commonly performed by the tuner/capture subsystem, which transfers the decoded data in digital form to the host over the expansion bus. Implementing these functions within the tuner subsystem, however, undesirably adds to the cost and complexity of the tuner subsystem. On the other hand, performing these functions on the host by providing the host with oversampled data would consume an undesirable amount of bus bandwidth and memory. What is needed, therefore, is a technique for processing teletext data, including anti-ghosting, which overcomes these and other problems.